Download Alternating Current * Learning Outcomes

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Alternating Current – Learning Outcomes
 Describe alternating current in terms of voltage and
current with time.
 Use an oscilloscope to show a.c.
 Discuss the use of a.c. in the national grid.
 HL: Solve problems about peak and rms values of
voltage and current.
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Alternating Current
 While in direct current (d.c.), current
always flows in the same direction
(positive to negative), alternating current
(a.c.) continuously changes directions.
 The current does not change direction
abruptly – it is a gradual process of
acceleration, fitting models of waves
and SHM.
 Thus when we graph a.c. it appears as a
sine wave.
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Alternating Current – Graphing
 For Ohmic conductors, 𝑉 ∝ 𝐼, so voltage-time and
current-time graphs look the same (though may have
different scales)
 We use lower case v and i to denote instantaneous
values for voltage and current.
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Oscilloscopes
 Oscilloscopes are instruments that show the variation in
electrical signals flowing through them over time.
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National Grid
 As mentioned in earlier, generators generate an emf by
electromagnetic induction.
 Due to the particular way turbines work (rotating coil in a
magnetic field), the emf generated is a.c.
 The voltage varies from +325 V to -325 V on the mains,
changing direction every 1/100 seconds (thus a full
cycle takes 1/50 seconds and the frequency is 50 Hz.
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Root-Mean-Square
 With d.c. we can perform calculations using a constant
current and voltage.
 Since current and voltage vary in a.c., various formulas
cannot be used normally.
 We use an “rms” value for a.c. voltage and current,
which are given by:
 𝐼𝑟𝑚𝑠 =
𝐼0
2
 𝑉𝑟𝑚𝑠 =
V0
2
 These are effectively average values, so we can discuss
average effects of a.c. over time.
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Root-Mean-Square
 e.g. Domestic electricity is supplied at an rms voltage of
230 volts. Find the maximum value of the voltage in any
one cycle.
 e.g. An alternating current flowing in a wire of resistance
10 Ω produces heat at a rate of 60 W. Find:
i.
the rms value of the current,
ii.
the peak value of the voltage across the wire.
 400 turn oil has a resistance of 200 Ω and is connected
to an a.c. supply. Over a 1 ms time interval the flux
through the coil increases by 5 × 10-4 Wb. Find the
average induced emf over the 1 ms interval. Find also
the average current if the average voltage is 300 V.